This invention relates to the field of drop marking systems of the type in which a liquid ink is forced under pressure through a nozzle which converts the liquid into droplets which can then be controlled by various means while projected toward a substrate for marking purposes. Examples of such systems include the familiar ink jet marking systems used for high speed label printing, product identification and the like, although there are other drop marking systems known in the art. One particular type of system which advantageously employs the present invention is the continuous stream, synchronous ink jet printer. Such a system typically includes an ink reservoir and a remotely located nozzle connected to the reservoir by a conduit. Ink is forced under pressure from the reservoir to the nozzle which emits a continuous stream of ink drops. The ink, which is electrically conductive, is provided with a charge as the drops leave the nozzle. The drops then pass through a deflection field which causes selected drops to be deflected so that some of the drops are deposited onto a substrate while the remaining drops are returned to the reservoir by a suitable ink return means.
It is known in the prior art to sense the flow of the ink from the reservoir and adjust ink parameters to maintain a desired flow rate. This teaching is found in the present assignee's prior U.S. Pat. No. 4,555,712. That patent, of which the present invention is an improvement, is hereby incorporated by reference. In the '712 patent a method and apparatus are disclosed which provide a means for determining and maintaining ink drop velocity substantially constant and does so in a manner which is substantially more accurate than was obtainable in the prior art.
In a preferred embodiment of the '712 patent the control system adjusts the flow rate by controlling the addition of make-up solvent to the ink reservoir. The viscosity of the ink is thereby adjusted so as to maintain drop velocity substantially constant.
Experience with this system has demonstrated that the percentage of solids (dyes and resins) in the ink composition varies, due to solvent addition, by as much as ten to forty percent from its initial composition in the course of the system operating to maintain substantially constant drop velocity as the temperature of the ink varies. Such a wide shift in composition affects other characteristics important in an ink jet system, such as ink drying time, drop break off point and even the charging characteristics of the ink drops. As a consequence, viscosity variations, due principally to temperature fluctuations during operation of the equipment, must be recognized by the control system so that solvent is added in a manner that does not excessively modify the formulation of the inks used in the system.
More specifically, present ink jet fluid control systems employ flow meters, of the type disclosed in the '712 patent, to control the addition of solvent to the ink. However, viscosity and, therefore, flow time, vary as a function of both compositional changes in the ink and temperature. The prior art system did not teach any correction for temperature variation. As a result, solvent may be added to the system when the flow time is too high, principally due to a temperature decrease rather than solvent loss. This can cause the aforementioned wide variation in the ink's composition resulting in undesirable operating characteristics. Conversely, solvent may be withheld from the system when the flow time is kept low by a temperature increase even though solvent may be needed as a result of evaporative losses due to system operation.
Accordingly, it is desired to provide a system which can compensate for both types of viscosity variations (compositional changes and temperature changes).
The present invention measures a change in temperature of the ink at selected intervals and calculates the flow time difference for this temperature change. The result is used to alter the reference flow time used to control the addition of solvent to the system. This results in elimination of the ambiguity due to temperature changes during system operation.
It is known in the art to employ a flow meter and a temperature sensor to determine a representative viscosity of a fluid, such as ink. Exemplary of this type of system is U.S. Pat. No. 4,714,931 to Erskine. As disclosed in connection with FIGS. 1 and 2 of that patent, the addition of solvent is controlled by a microprocessor which receives as inputs flow data from a viscometer 12, pressure from a transducer and temperature data. The Erskine patent, however, employs temperature and pressure values stored in a look up table resident in a read only memory (ROM) to provide reference flow times.
Depending upon the actual temperature and pressure detected, a specific flow time reference value is accessed from the ROM and used by the microprocessor system to control addition of solvent. Such a system cannot take into account the many variations in initial ink viscosity, calibration settings, capillary dimensions, and other system parameters which affect flow time and which differ from installation to installation for the same system or different printer systems of a similar type. Furthermore, the Erskine system depends upon absolute temperature and pressure values and, therefore, inaccuracies, due to the miscalibration of the temperature or pressure sensor, can interfere with the intended operation of the system.
It is accordingly an object of the present invention to provide an improved control system related to the method and apparatus disclosed in U.S. Pat. No. 4,555,712 whereby the effects of temperature variation during system operation can be accounted for.
It is a further object of the present invention to provide a feedback control for a fluid delivery system in which both flow time and temperature are monitored whereby the desired properties of the fluid can be maintained substantially constant by selective adjustment of the flow time.
A further object of the invention is to provide a system of the type described in which temperature differences are employed rather than absolute temperature values, whereby inaccuracies due to miscalibration of the temperature sensor are eliminated.
It is a further object of the invention to provide a dynamic control system which can take into account flow time differences between identical systems due, for example, to manufacturing tolerances or to initial set-up variations. Such a dynamic system periodically recalculates a reference flow time based on a particular system's operating characteristics. System to system variations are, therefore, irrelevant because only flow time and temperature differences relative to initial or preceding values are considered.
These and other objects and advantages of the invention will be apparent from the remaining portion of the specification.